Go to your Library and find Grand Theft Auto IV in your game list on the left. Right-click on the game title and select Properties. In the Properties window, look for the section called Launch Options right in the middle, and enter the following in the field:


This forces the game to recognize 2GB (2048MB) of video memory. You may also tweak the value (2048) based on your actual VRAM. As long as the game doesn't try to use more VRAM than your GPU actually has you should have no issues. Keep in mind the game only uses 1417 MB of VRAM in 4K (2160p) with maxed out settings.